The present invention relates to the method of automatically carrying out pattern recognition or pattern measurement, in the field of optical information processing and optical measurement, by applying optical correlation processing using coherent light to various two-dimensional images produced from an imaging device such as a CCD camera.
A joint transform correlator is generally used in the conventional optical pattern recognition apparatus and the conventional correlation processing apparatus. For example, FIG. 4 shows a conventional construction using an optically writeable spatial modulator. In this example, an input device 63 produces a composite input image composed of a reference image. for use as a recognition reference and an object image disposed adjacently to the reference image for correlative comparison. A laser source 50 emits a laser beam which is expanded by a beam expander 51, and which is thereafter divided by a beam splitter 52 into two beam components. One beam component passing the beam splitter 52 irradiates the input device 63 to convert the composite input image into a coherent image. This coherent image is converted by a first Fourier transform lens 29 into a joint Fourier-transformed image of the reference image and object image, which is recorded in the form of an optical density distribution on an optically writeable liquid crystal light valve 55 disposed on a Fourier image plane of the lens 29.
The other beam component reflected by the beam splitter 52 is sequentially reflected by mirrors 58, 59 and a polarization beam splitter 56 to thereby irradiate the light valve 55 rearward to reproduce the recorded optical density distribution of the joint Fourier-transformed image in the form of a coherent image. This coherent image passes through the polarization beam splitter 56, which functions as an analyzer, to form a corresponding negative or positive image. This image is again Fourier-transformed by a second Fourier transform lens 34, and is then received by a CCD camera 57 disposed on a Fourier image plane of the lens 34. By such construction, there is obtained a correlation peak which indicates a two-dimensional correlation coefficient of the object image and the reference image. A spot position of the correlation peak is uniquely determined according to a relative positional relation between the reference image and the object image around the optical axis of the system.
The above noted construction utilizes an optically writeable liquid crystal light valve of the reflection type as a spatial modulator or image recorder. Otherwise, there can be used an image recorder of the transmission type such as a BSO crystal (BiSiO) modulator. Further, in place of the optically writeable spatial modulator, a joint Fourier-transformed image is once detected by a CCD camera, and then is displayed on an electrically writeable spatial modulator such as a television monitor.
FIG. 5 shows an example of a composite input image composed of adjacent object and reference images. FIG. 6 shows a correlation peak pair indicative of a two-dimensional correlation coefficient between the reference and object images. However, in such system, it would be difficult to improve the correlation processing by concurrently effecting correlation processing between a single object image and a plurality of reference images which are a data base for correlation analysis, because of various reasons such as degradation of S/N ratio.
In order to solve the above noted difficulty, the inventors have proposed a joint transform correlator having a feedback loop, which is disclosed in Japanese Patent Application Nos. 167758/1989, 155224/1989, 247612/1989, 5241/1990 and 5246/1990, and corresponding U.S. application Ser. No. 539725. In this feedback system, at the first step, a composite input image is formed of a single object image and a plurality of reference images. The composite input image is subjected to the correlation processing to produce correlation peaks which indicate respective two-dimensional correlation coefficients between the object image and the respective one of the reference images. The optical intensity of the respective correlation peaks are measured individually. At the second step, respective intensity values of the peaks are normalized by a maximum intensity value of the strongest peak. At the third step, a beam component passing through each reference image is regulated in linear or nonlinear relation to a corresponding normalized value through a masking spatial modulator which is disposed forward or rearward of the reference images and which is driven gradatively according to the normalized values. By such operation, the correlation coefficient values are feedback to regulate intensity of the beam components passing the respective reference images, and this feedback operation is repeatedly carried out to discriminate the most significant correlation peak.
However, in the above described joint transform correlator of the feedback type, pattern recognition operation can be applied only to a limited number of reference images at one time. For example, if the reference images are alphabet characters, a maximum of thirteen characters can be concurrently processed. If more than that number of characters were processed concurrently, erroneous recognition may result. Further, it would be practically difficult to use a greater number of reference images, because the composite image input device has a limited display area. Alternatively, a composite image input device having an enlarged display area might be utilized to increase a number of reference images to be inputted concurrently. However, such modification causes a scale-up of the overall construction of the optical system and increase in cost. Further, the size of an individual reference image might be reduced for efficient use of the display area. However, in such case, a reduced reference image cannot be electrically inputted by a typical spatial modulator. Instead, noneraseable input means, such as photographic film, is adopted to treat a reduced reference image, thereby limiting a total number of reference images. Therefore the prior attempt at a feedback type joint transform correlator has the drawback that efficient pattern recognition is not effected among a great number of images such as characters.